Energy-Dependent Neutron Transport

Abstract

Fast neutrons generally lose energy when they undergo collisions with nuclei. When the incident neutron energy exceeds the lowest excitation level of the target nucleus such interactions can be inelastic. In this case the target nucleus absorbs some of the neutron’s kinetic energy and the system kinetic energy is not conserved in the interaction. Therefore the energy loss may be uncorrelated with the scattering angle. However, under some conditions the internal energy of the target nucleus may not be changed by the neutron interaction. Classically this occurs when the neutron energy is below the lowest excitation energy of the target nucleus. In this case the scattering is elastic and kinetic energy is conserved. In many materials the slowing down of neutrons from a few hundred keV to thermal energies may be described to a reasonable degree of accuracy by the physics of elastic collisions. For hydrogen, this range of validity extends from a few MeV to thermal energies.